| Document and Entity Information | Oct. 22, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 22, 2020 |
| Entity Registrant Name | COUNTY BANCORP, INC. |
| Entity Central Index Key | 0001470205 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity File Number | 001-36808 |
| Entity Incorporation, State or Country Code | WI |
| Entity Tax Identification Number | 39-1850431 |
| Entity Address, Address Line One | 2400 South 44th Street |
| Entity Address, City or Town | Manitowoc |
| Entity Address, State or Province | WI |
| Entity Address, Postal Zip Code | 54221 |
| City Area Code | 920 |
| Local Phone Number | 686-9998 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, $0.01 par value |
| Trading Symbol | ICBK |
| Security Exchange Name | NASDAQ |
